This invention relates to methods for in situ immobilization of metals in water and earthen boundaries bordering the water as well as to immobilization treatment of metals in water having varying density zones to access and treat all regions within the water and at the water-earthen boundaries.
Waste stacks are generated by many types of industrial processes, often as a result of the extraction of valuable materials. The waste stacks are frequently piles of economically invaluable material left over from the industrial processes. For instance, power plants often generate waste stacks of ash. The ash is left over when energy is extracted from fuel by burning. Mining processes also often generate waste stacks. The waste stacks contain minerals left over after a valuable metal or mineral is extracted from the mined earth materials. For example, phosphorus mines often result in waste stacks containing predominantly gypsum as a processed waste. The waste stack gypsum is a relatively invaluable mineral left over after phosphorus is removed from the mined materials.
In many instances, waste stacks are formed as follows. First, the residual or waste material is combined with water to form a waste slurry. This waste slurry is then flowed to a settling pond where the solids contained in the waste slurry settle out. Water evaporates or permeates from the settling pond. Over time, the settled solids leave behind a stack of waste material. Some water is retained in the settled waste material which makes up the waste stack. This process of deposition settling and evaporation is repeated until the resulting waste stack is too large for the process to economically continue, or is terminated for other reasons. If needed, a new waste stack is started and grows in a similar fashion. FIG. 1 shows a mine 12 which has been in operation for a significant period of time and is surrounded by a number of waste stacks 14. The individual waste stacks 14 are often huge, frequently comprising millions of cubic yards. The amount of material currently stored in waste stacks is enormous, and it continues to increase as mining and other industries continue to produce and develop new operations.
A problem associated with waste stacks is toxic metal migration. The actual percentage of water-soluble toxic metals in a given waste stack is usually very small; for example, less than 1.0 percent. However, because the waste stacks are often very large, the total amount of toxic materials in a waste stack is often large enough to present some risk to surrounding areas and ground water. These risks arise in part due to potential metals migration of liquids from the waste stack. The slurry water may percolate into the soil in addition to evaporating or remaining in the waste stack.
Toxic metals potentially found in waste stacks include but are not limited to Pb, Hg, U, Cd, Fe, As, Se, Cu, Cr, Ni, Zn, Co, Mn, and Ag. Over time such metals can leach out of the waste stacks and into ground water. Thus, it is desirable to keep the metals within or near the waste stacks to minimize the danger posed by such metals.
Keeping the metals within or near the stack is often difficult, especially since the metals may be present in water-soluble forms. Such water-soluble forms can migrate as metal solutes whenever water moves through the stacks. Since the stacks are frequently exposed to water, either in the form of rain or in the form of wastewater deposited on the stacks, water-soluble metals or metal compounds present in the stacks are exposed to conditions that may encourage their migration. In some situations, metals have already begun migrating out of existing waste stacks and into a boundary zone or layer below the waste stacks. Thus, it is desirable to have a method which will not only inhibit further migration of metals from the waste stacks, but which will also inhibit the migration of metals that are in a boundary layer beneath the waste stacks.
One method for containing metals within a waste stack has been to treat the waste stack with microbes that are capable of producing microbial sulfides. The microbial sulfides are sulfide byproducts of microbial activity in waste stack affected zones. The microbial sulfides react with metal ions or metal containing compounds contained in the waste stack affected zones to form metal sulfides. The metal ions or metal containing compounds contained in the waste stack affected zones become relatively insoluble during this treatment and are inhibited from migrating within or from the waste stack affected zone. This method was first disclosed in U.S. Pat. No. 5,632,715, which is incorporated herein by reference. This method has been applied successfully in treating waste stacks and inhibiting migration of metals within boundary areas within such waste stacks.
With the excavation of mining materials during the mining process, a void is typically created that is often filled with water. These large water-filled zones, typically known as pit lakes 15, contain many of the same type of contaminants as are found in waste stacks, especially when the lake is adjacent a waste stack from the mining operation. Additionally, the pit lakes have soil boundaries along the surface as well as extending down to the bottom of the lake. Metal migration continues to occur within the soil boundaries between the water and the soil. Further still, the water from the pit lakes can seep into adjacent water tables, which can result in the contamination of water systems in populated areas.
One prior art method of treating such bodies of water has been to pump the water from the lake source to a process treatment plant and then return the treated water to the pit lake. Another method in the prior art for treating such bodies of water has included taking the process treatment plant to the body of water and placing it on a boat that travels across the surface of the pit lake to treat the water at the surface level and return the treated water back to the surface.
There are several problems that exist in either solution. Firstly, both treatment solutions are expensive to conduct, as the cost of pumping the water alone can be extreme. Secondly, mixing treated water with contaminated water only causes the contaminated clean water to be re-contaminated, or require there be a secondary storage facility, which is not always available or suffers from the same soil contamination of the first pit lake. Thirdly, the plant operators must be on the water in the process on the lake method, which potentially exposes the operators to the contaminated water unnecessarily. Fourthly, these treatment solutions are at times unable to reach the depths of these pit lakes, which can have depths ranging from 50 feet to as great as 3,000 feet. Again, water pumping becomes expensive for deep pit lakes. Fifthly, certain water-filled workings are completely subterranean and are virtually impossible to access directly or the water is so deep that pumping the water from the subterranean cavity to the surface for treatment becomes cost prohibitive.
Not only must the water be treated in such conditions, but so to must the soil boundary within either the subterranean pit lake or the open body pit lake also be treated during the treatment process. The prior art methods of removing the water for treatment at a separate location, or merely treating the water on the surface of the pit lake fails to treat to treat the soil boundary of the lake simultaneously with treating the water.
Accordingly, there is a need within the industry to be able to treat contaminated water sources, such as pit lakes and subterranean mine cavities filled with water, in an economical and environmentally sound way that also includes treating the soil boundaries adjacent the water.
According to the present invention, an in situ method for treating large bodies of water having varying density strata to immobilize contaminant metals within the water is disclosed. The method is also able to treat the soil water boundary within the pit lake to provide additional immobilization. The pit lakes can include open pit lakes, subterranean mine lakes, flowing streams and the like. The method is also able to treat an abandoned mine prior to the filling of the mine with water. The invention also contemplates treating bodies of water having varying density strata.
The method, or process for in situ immobilization of metals is focused on treating large bodies of water having metals therein that are also adjacent a border of soil or earthen materials in an attempt to immobilize the metals from penetrating through the soil. Initially, the density mean of the body of water is determined, which is densest typical at regions at or approaching 4 degrees C. The process includes introducing a treatment substance that has a density greater than that of the density means into the body of water, providing at least one microbe proximate or in the body of water, producing microbial sulfides arising from the initial microbe placement, causing microbial sulfides to react in situ with metal ions or metal containing compounds located within the body of water, reducing the solubility of the metal ions by forming metal sulfides, and inhibiting the migration rate of the metal ions or other metal containing compounds within or from the soils or earthen materials as they settle out of the water. The treatment substance typically includes at least one microbe nutrient to sustain activity of the microbes added thereto. The microbial activity yields microbial sulfides that react with the contaminants within the water to form the metal sulfides.
The treatment can include more than one supplemental feeding of the treatment substance and the treatment substance can be either in liquid or powder form, which dry powder form may include pellets ranging in the size from one millimeter to 300 millimeters in diameter. The pellets, in larger size form, can be processed to have an average density larger than the density mean of the water so that the weigh of the pellets carries them past the densest regions within the water and dissolve at a rate suitable for dispersal of the treatment substance throughout the body of water.
The treatment substance, or fluid, is also buffered so as to balance the pH of the water being treated within a range of 6 to 8 pH. Accordingly, the treatment substance includes a treatment fluid having a pH range of about 1 to 12 in order to buffer the water during application. The microbes that are relied upon to generate the microbial sulfides can also occur naturally within the body of water or within the soils or earthen materials.
The treatment substance is also designed to specifically exclude cysteine. The sulfides typically react with contaminant metals including AS, SE, CD, HG, CU, CR, U, FE, ZN, PB, NI, CO, MN, and AG.
The treatment substance is further characterized to include a concentration of a carbohydrates to serve as microbial nutrients. The carbohydrate has a concentration of up to 10 grams per liter of fluid to be treated and can further include up to 0.1 grams of total nitrogen per liter of fluid to be treated. Further still, the treatment substance can also include about 0.25 grams of phosphate ion per liter of fluid to be treated or a combination of carbohydrate, phosphate ion, and total nitrogen. The phosphate can be adjusted by volume weight to carry the treatment substance below the densest regions within the body of water. The treatment substance can also include buoyant agents that carry the nutrients from lower first regions to higher second regions after the treatment substance reaches the first region, which is typically below the densest regions. This buoyant agent can be biologically derived, chemically derived, or be a gas, such as one selected from, but not limited to, N2, CO, CO2, H2, CH4, SO2, H2S.
In an alternative embodiment, the treatment substance comprises one or more alcohols and a carbohydrate, which can be selected from the group of whey, corn sirup, or hydrolyzed starch. in the alcohol and carbohydrate mixture, the treatment substance has generally a 3:1 ratio of alcohol to carbohydrate and can also include up to 30 mg. of total nitrogen per liter of liquid to be treated. The microbe can be selected to include one or more microbes selected of a genus coming from the group consisting of Desulfovibrio, Desulfomonas, and Desulfomaculum.